An older man presents with chest pain and a unilateral vesicular-bullous rash in a dermatomal distribution on the chest. What is the most likely diagnosis?

• A. Pityriasis rosea
• B. Bullous pemphigoid
• C. Contact dermatitis
• D. Herpes zoster

Answer: (D)

Reactivation of varicella-zoster virus in a sensory ganglion causes shingles, which presents as a unilateral, vesicular eruption that follows a single dermatomal distribution. In this scenario, the chest area is innervated in a belt-like pattern by thoracic nerves, so the vesicles appear in a band on one side of the chest and do not cross the midline. Pain or a burning sensation often precedes the rash, reflecting nerve involvement.

This pattern helps distinguish it from other common skin conditions. Pityriasis rosea typically starts with a herald patch and then a symmetric, oval rash along skin lines rather than a sharp dermatomal band. Bullous pemphigoid presents with large, tense bullae more diffusely on the skin and lacks the classic dermatomal distribution. Contact dermatitis is localized to areas of contact exposure and is usually eczematous with pruritus, not aligned to a dermatome.

So, the unilateral vesicular rash in a dermatomal pattern on the chest in an older adult points most strongly to herpes zoster.